(1) Field of the Invention
The present invention relates to a mobile station for a Code Division Multiple Access (CDMA) communication system and a method for communication of the mobile station.
(2) Description of the Related Art
Recently, Code Division Multiple Access (CDMA) systems have been widely used in cellular phone systems. In the CDMA system, a transmitting station spread modulates a signal using special codes of pseudorandom noise (PN) sequence to transmit via broadband spectrum spread, while a receiving station reconstructs required signals by reverse-spread modulated using the same PN codes as those on the transmission side. Here, this PN sequence has a property of orthogonality whereby the cross-correlation value with the other sequence becomes zero. Due to the property of PN sequences, even when a plurality of transmission stations transmit spread modulated signals using different PN sequences, a receiving station can separate and extract signals only from desired transmission station using the same PN sequence used in the required transmission station.
The following are the methods for switching over to another base station with which to communicate. Firstly, base stations spread modulate and transmit pilot signals (control signals) using the PN sequence specific to each base station and which the mobile station uses for searching for a communication target. The offset values of the PN sequence are called PN numbers and range from 0 to 511. Since each base station can be identified with this PN number, the base station using the PN series whose PN number (offset value) is n is called a “base station n.” The communicating base station is called an “active base station.” The active base station transmits a neighbor list to the mobile station, the list indicating the numbers of the base stations located near the active base station. Each base station included in the neighbor list is called a “neighbor base station”. Other base stations, other than the active base station and the neighbor base stations, and located far from them are called “remaining base stations.” The active base station also transmits a remaining search interval number (R_INC) which indicates an interval between PN numbers of remaining base station to be searched and which the mobile station uses for searching a communication target.
The mobile station, during communication, always searches for a better base station than the currently active base station. That is, the mobile station measures strengths of signals transmitted from remaining base stations whose PN numbers apply to each R_INC, and from all neighbor base stations. Then, if any base station whose signal strength is larger than that from the active base station is detected, the mobile station hands off to the detected base station. Therefore, naturally the mobile station may hand off to one of the remaining base stations.
However, in case of a hand-off to remaining base stations, there are the following disadvantages.
The first disadvantage is that signals from the mobile station (i.e., reverse signals) have trouble reaching remaining base stations, because remaining base stations usually are located far from the mobile station. This is because, unlike base stations, the mobile station cannot transmit a signal having large energy value. Therefore, even if a hand-off is performed to one of the remaining base stations, communications may not be established between the remaining base station and the mobile station. Alternatively, even if such communications can be established, the communication quality may substantially be bad.
The second disadvantage is as follows: that is, in case of a hand-off to one of the remaining base station located far from the mobile station, the remaining base station becomes an active base station, thereby the neighbor base stations near then active base station also become remote base stations. In this case, since the base stations near the current location of the mobile station become remaining base stations, such base stations will not always become a target for the next search, because remaining base stations are searched skipping some stations according to the R_INC. As a result of that, though the strength of the signal transmitted from the base stations near the mobile station is usually larger than that from remote base stations, a hand-off to these near base stations becomes difficult.
There are many disadvantages in case of a hand-off to remaining base stations as above, so it is desirable to avoid the hand-off to remaining base stations as much as possible.
Meanwhile, as the mobile station moves, phasing may occur, where the propagation condition of radio waves changes and the strength of received signals fluctuates. Especially, remaining base stations are located far from the mobile station and their signals pass through various propagation conditions, so that their signal strengths are susceptible to time fluctuation. Consequently, the strength of signals from remaining base stations may be larger at the measuring time only. Thus, there is no guarantee that it is continuously larger at times other than the measuring time.
In the conventional hand-off methods to base stations, however, once the strength of the signal transmitted from one of the remaining base stations is detected to be larger than that from the active base station, then the mobile station hands off to the remaining base station regardless of whether the signals strength from the remaining base station is continuously larger at times other than that measuring time or not.